Programmable, touch probe coordinate measuring machines are used for inspections of machined metal parts to insure compliance with design parameters. These machines move a probe through a predetermined path, under direction of a numerical control system, to contact workpiece surfaces. They may be used to determine the centerline of a bored holes and the distances between the centerlines of two bored holes by contacting three or more spaced points on the interior diameter of the hole and using those dimensions to calculate the center line. However, when the hole is threaded this contact technique is impractical. To overcome this problem, hole location plugs have been developed which incorporate a threaded section adapted to screw into the hole and a concentric cylindrical stem which extends above the hole. The coordinate measurement machine is programmed to contact three points on the outer diameter of the stem and use the coordinates of these points to calculate the center line of the stem and thus of the threaded hole.
Two problems are encountered in the use of these stemmed hole location plugs. First, in order to contact three spaced points on the outer diameter of the stem the system must be programmed to cause the touch probe to undergo at least six movements: the probe must be moved into contact with one point of the stem and then moved out of contact before it can be moved into contact with a second point. These multiple motions substantially slow the process of the inspection of bored holes using the stem-type plugs. Second, in order to provide sufficient clearance for the probe motion without contacting other parts of the workpiece surface, the stem must extend a fair distance above the workpiece surface, typically an inch or so. If the relatively thin plug stem is not absolutely concentric with the plug section that threads into the hole to be measured, the precision of the centerline measurement will be diminished.